The present invention relates generally to digital audio broadcasting (DAB) and other types of digital communication systems, and more particularly, to interleaver synchronization techniques for such digital audio broadcasting and other types of digital communication systems.
Proposed systems for providing digital audio broadcasting in the FM radio band are expected to provide near CD-quality audio, data services and more robust coverage than existing analog FM transmissions. However, until such time as a transition to all-digital DAB can be achieved, many broadcasters require an intermediate solution in which the analog and digital signals can be transmitted simultaneously within the same licensed band. Such systems are typically referred to as hybrid in-band on-channel (HIBOC) DAB systems, and are being developed for both the FM and AM radio bands.
In order to prevent significant distortion in conventional analog FM receivers, the digital signal in a typical FM HIBOC DAB system is, for example, transmitted in two side bands, one on either side of the analog FM host signal, using orthogonal frequency division multiplexing sub-carriers. In an OFDM communication system, the digital signal is modulated to a plurality of small sub-carrier frequencies that are then transmitted in parallel.
In the United States, the frequency plan established by current FCC regulations separates each transmitting station in a geographical area by 800 KHz. Any transmitting stations in adjacent geographical areas, however, are separated from a local transmitting station by only 200 KHz. Thus, a particularly significant source of interference in such a system is known as first adjacent analog FM interference. This interference results when a portion of a FM host carrier in an adjacent geographic area overlaps in frequency with a portion of a digital signal side band. Although first adjacent analog FM interference, when present, typically affects only one of the two digital side bands, it nonetheless represents a limiting factor on the performance of DAB systems. The presence of a strong first adjacent interference signal will significantly degrade the performance of the digital signal transmissions, even when one of the two side bands is free from interference.
Symbol interleavers are employed in many communication systems. Interleaving scrambles a signal over a certain time interval, by reordering the data bits. Typically, block interleavers are employed, where a signal is scrambled by writing the symbols into rows and reading them out in columns, in a known manner. If block-coded symbols are interleaved over the duration of many blocks before transmission, symbols associated with a lost packet will be de-interleaved by the receiver and found among many different coded blocks. Thus, the number of symbol errors that may occur in each coded block is reduced, and the likelihood that a selected block code will correct all symbol errors in a transmitted signal is correspondingly increased.
In OFDM-based communication systems, and especially in the IBOC case, the de-interleaver at the receiver has to be synchronized to the interleaver. Typically, interleaver synchronization is performed by inserting a unique bit pattern into the data stream, thereby requiring additional channel bandwidth. Such interleaver synchronization mechanisms, however, result in delay, overhead information and additional processing. A need therefore exists for an interleaving method and apparatus for an OFDM-based communication system that eliminates additional overhead information and reduces the complexity and processing for symbol interleaving.
Generally, interleaving methods and apparatus are disclosed for an in-band on-channel (IBOC) digital audio broadcasting system. To overcome the effects of intersymbol interference, any two adjacent symbols in a conventional orthogonal frequency division multiplexing system are separated by a guard period. It is known to use the guard period to provide a mechanism for OFDM frame synchronization.
According to one aspect of the invention, the guard period of successive OFDM frames is utilized to establish one or more unique positive or negative patterns and thereby provide a novel method for interleaver synchronization. In this manner, by proper positioning of the guard period patterns, the present invention can identify one or more particular portions of each interleaver block, such as the beginning and midpoint of each interleaver block.
In one illustrative implementation, OFDM frame synchronization is achieved by retransmitting the last 16 samples of each symbol, during the guard prefix, referred to as the cyclic prefix. Thereafter, the 512 samples of user data are transmitted during the useful symbol period. Finally, the first 16 samples of each symbol, are retransmitted during the guard suffix, referred to as the cyclic suffix. If the phase of the cyclic prefix and cyclic suffix, a1 and b1, is the same, it is referred to as a positive prefix. If the phase is opposite, it is referred to as a negative prefix.
The present invention can identify the beginning of each interleaver block, for example, by positioning the fourth negative cyclic prefix with OFDM frame zero (0). In addition, the present invention can optionally identify the midpoint of each interleaver block by positioning the fourth negative cyclic prefix with OFDM frame 206 (assuming 414 frames, numbered 0 through 413). The beginning of an interleaver block can be distinguished from the midpoint using a unique cyclic prefix pattern. For example, the beginning of each interleaver block can be identified by a unique cyclic prefix pattern comprised of xe2x80x9c-+-++-+++-,xe2x80x9d while the midpoint of each interleaver block can be identified by a unique cyclic prefix pattern comprised of xe2x80x9c-+++-++++-+++++-.xe2x80x9d
According to another aspect of the invention, a receiver can detect when synchronization is lost, and detect and correct a false synchronization. Once a predefined synchronization condition is satisfied (i.e., a unique positive or negative pattern is established using the guard periods), a synchronization (SYNCH) state is entered. While in the synchronization state, each received frame is monitored for the synchronizing pattern to ensure that the receiver remains in the SYNCH state. If synchronization remains correct, the next synchronization pattern should appear at the periodic OFDM frame interval (every 414 frames). If the pattern is not detected at the expected point, synchronization is at least temporarily lost. After a temporary loss of synchronization, if the synchronization pattern is detected at the expected position, the temporary loss of synchronization condition is ignored, and the receiver returns to the synchronization (SYNCH) state. If the synchronization pattern is not detected at the expected position for a predefined number of blocks, synchronization is lost and the receiver must monitor each received frame for the synchronizing pattern. If a synchronization pattern is detected at an unexpected position a predefined number of times, while in the synchronization state, synchronization is lost and the receiver must search for the synchronizing pattern all over again.